1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, or a facsimile machine that forms an image using a two component developer made up of toner and a magnetic carrier, and an image density control method.
2. Description of the Related Art
The two component development system is a conventionally known technique that involves supporting a two component developer made of toner and a magnetic carrier (simply referred to as ‘developer’ hereinafter) on a developer carrier, forming a magnetic brush with the magnetic developer by the magnetic pole provided within the developer carrier, and developing a latent image on a latent image carrier with the magnetic brush. The two component development system is widely used since it enables color imaging with relative ease.
It is noted that in forming an image, an image forming apparatus has to maintain a constant image density. The image density is determined primarily by the developing capacity of a developing apparatus. The developing capacity represents the amount of toner adhered to a latent image in the development process. The developing capacity may vary depending on the toner density of the developer contained in the developing apparatus, developing conditions such as the developing potential representing the potential difference between the latent image formed on the surface of the latent image carrier and the developer carrier surface on which a developing bias is applied, and the amount of charge of the toner used for image development for example. Accordingly, a typical image forming apparatus is configured to control image density control conditions such as toner density, writing light, and developing bias in order to maintain a constant image density. For example, the toner density of the developer within the developing apparatus may be controlled by controlling toner supply operations based on an output value of a toner density detecting unit that detects and outputs the toner density so that the toner density of the developer may be equal to a toner density control reference value. Also, the developing potential may be controlled by obtaining a suitable developing potential for maintaining a constant image density based on the slope of a relational expression representing the toner adhering amount in relation to the developing potential (γ development) and controlling the writing light and the developing bias so that the developing potential may be equal to the suitable developing potential.
As can be appreciated, it is relatively easy to control image density control conditions such as toner density and developing potential (writing light and developing bias) may be controlled to predetermined target values for obtaining a desired image density. However, it is relatively difficult to control the amount of charge of the toner used in the image development to a predetermined target value for obtaining a desired image density. As a consequence, stable developing capacity may not be achieved even if the developing potential and the toner density may be maintained constant and in turn, a desired image density may not be obtained.
Specifically, for example, in the case of outputting an image with a low image area ratio, the amount of toner consumed upon developing such an image may be relatively small so that the amount of toner to be supplied to maintain the toner density to a desired density level may be relatively small. In this case, toner is likely to reside within the developing apparatus for a relatively long time. Toner residing within the developing apparatus for a relatively long time may be stirred for a long time so that a large portion of the toner used in image development may have a charge that is greater than the desired charge. In turn, a strong electrostatic force may be required for separating the toner from the carrier so that the developing capacity may be degraded. On the other hand, in the case of outputting an image with a high image area ratio, a large portion of the toner residing within the developing apparatus may be new toner that has just been supplied and is not yet adequately charged so that a large portion of the toner used in image development may not be charged to the desired charge level. In this case, the electrostatic force required for separating the toner from the carrier may be relatively weak so that the developing capacity may be relatively high. With the growing demand for miniaturization of the developing apparatus, the amount of developer contained within the developing apparatus is being reduced. In turn, there is a growing number of instances in which toner used for image development is not adequately charged to the desired charge level during image formation performed after outputting an image with a high image area ratio. Therefore, the developing capacity during image formation performed after outputting an image with a high image area ratio tends to be relatively high.
As can be appreciated, the proportional amount of newly supplied toner residing within the development apparatus after image output may vary depending on whether an image with a low image area ratio is successively output or an image with a high image area ratio is successively output, for example. In turn, differences are created in the developing capacity. That is, the developing capacity cannot be maintained constant even when the developing potential and the toner density are maintained constant so that a fixed image density cannot be achieved.
It is noted that an image forming apparatus that is configured to counter such a problem is disclosed in Japanese Laid-Open Patent Publication No. 57-136667 and Japanese Laid-Open Patent Publication No. 2-34877, for example. Such an image forming apparatus includes toner density detection means for detecting and outputting the toner density of a two component developer within a developing apparatus corresponding to an image density control condition and is configured to compare the output value of the toner density detection means with a toner density control reference value, control operations of a toner supply apparatus based on the comparison result, and control the toner density of the two component toner within the developing apparatus to a desired toner density. The image forming apparatus is also configured to detect the density of a reference toner pattern formed at a non-image region to determine the image density obtained at the time of forming the reference toner pattern and correct the toner density control reference value based on the detection result. By implementing such a technique, image formation with the desired image density may be enabled for some time after such correction is performed. That is, by periodically forming a reference toner pattern and correcting the toner density control reference value according to detection results of the reference toner pattern, a fixed image density may be achieved.
However, in the image forming apparatus according to Japanese Laid-Open Patent Publication No. 57-136667 or Japanese Laid-Open Patent Publication No. 2-34877, a reference toner pattern has to be formed each time the toner density control reference value is to be corrected. Therefore, the amount of toner consumed for operations other than image formation may be increased.
In view of such a problem, the inventors of the present invention has previously disclosed an image forming apparatus as is described in Japanese Laid-Open Patent Publication No. 2007-133235. Specifically, the image forming apparatus according to this disclosure includes information detection means for detecting information for determining the amount of toner exchanged at the developing apparatus within a predetermined period of time such as the image area ratio of an image output during this time period. In this way, the proportional amount of newly supplied toner and/or the proportional amount of old toner residing within the developing apparatus may be determined based on the detection result obtained by the information detection means so that the developing capacity of the developing apparatus may be determined. In turn, the toner density control reference value may be corrected by toner density control reference value correction means based on the detection result of the information detection means so that the toner density within the developing apparatus may be adjusted and a constant image density may be obtained. It is noted that the information on the toner exchange amount used for correcting the toner density control reference value in the above-described image forming apparatus may be detected without consuming toner unlike the case of detecting the image area ratio of an output image. Therefore, the amount of toner consumed for operations other than image formation may be prevented from increasing.
However, the image forming apparatus according to Japanese Laid-Open Patent Publication No. 2007-133235 does not have measures for responding to factors other than the amount of toner exchanged at the developing apparatus within a predetermined period of time. For example, the above-described apparatus is not configured to respond to fluctuations in the developing capacity of the developing apparatus due to environmental change or variations in standby time so that image density may still not be adequately controlled.
It is noted that occurrence of the above-described problems is not limited to the case of correcting the toner density control reference value for the toner density as the image density control condition based on the toner exchange amount information. For example, similar problems may occur in a case where the control reference values for toner density and developing bias as image density control conditions are fixed while a control reference value for the writing light as another image density control condition is corrected based on the toner exchange amount information. Also, similar problems may occur in a case where the control reference values for toner density and writing light as image density control conditions are fixed while a control reference value for the developing bias as another image density control condition is corrected based on the toner exchange amount information.